1. Field of the Invention
This invention relates to an excitation control apparatus for a synchronous machine.
2. Description of Related Art
FIG. 1 is a structural block diagram of a general excitation control system employed so far and disclosed, for example, in Japanese Patent Application Laid-Open No 58-176000 (1983). Such a system is represented in FIG. 1, where reference numerals respectively represent: 1 a synchronous machine, 2 field windings of the machine, 3 an exciter for feeding a field current to the field windings 2, 4 a potential transformer (PT) for detecting the voltage of the synchronous machine, 5 a current transformer (CT) for detecting the current of the synchronous machine 1, and 6 an automatic voltage regulator (AVR) for controlling the voltage of the synchronous machine 1 to be constant. The AVR 6 is specifically constituted of a voltage detection circuit 61 which detects the output of the PT4, a reference circuit 62 which generates a desired control value of the voltage of the synchronous machine 1, an amplifier circuit A 63 which amplifies the difference between the reference circuit 62 and the voltage detection circuit 61, a phase compensation circuit 64 which enhances the controlling stability of the voltage, an adding circuit 65 which adds signals from the other supplemental functional devices to be described later, and an amplifier circuit B 66 which amplifies the output of the adding circuit 65 and generates a signal to the exciter 3.
A power system stabilizer 7 detects the electric power from output signals of the PT 4 and CT 5 and supplies a supplementary signal to the AVR 6 thereby enhancing the stability of the system. Meanwhile, a V/Hz limiter 8 detects the V/Hz value(the ratio of the voltage V to the frequency Hz) from the output signals of the PT 4 and CT 5, feeds a supplementary signal to the AVR 6 and controls the V/Hz not to exceed a predetermined value. A power excitation limiter 9, by detecting the power and reactive power from the output signals of the PT 4 and CT 5, supplies a supplementary signal to the AVR 6 so as to drive the synchronous machine 1 at not lower than a predetermined level. The current flowing in the field windings 2 is detected by a shunt 10 which in turn outputs a signal to an overexcitation limiter 11. When the overexcitation limiter 11 detects the overexciting state not smaller than a predetermined value, it supplies a supplementary signal to the AVR 6 thereby to control to restrict the overexcitation.
The operation of the conventional controlling system in the structure described hereinbefore will now be depicted. The output voltage from the synchronous machine 1 is decreased by the PT 4. The signal of the PT 4 is inputted to the AVR 6 to be converted to a signal easy to amplify add, etc., (converted generally to a direct current signal) by the voltage detection circuit 61. The value from the reference circuit 62 is a desired control value for the AVR 6 to control the voltage of the synchronous machine 1. The output difference of the reference circuit 62 from the voltage detection circuit 61 is amplified to an appropriate value by the amplifier circuit A 63. If the amplified signal is positive, the output voltage of the synchronous machine 1 is lower than the desired control value of the voltage set by the reference circuit 62, and therefore, the AVR 6 outputs a control signal to raise the output of the exciter 3. As a result, the current flowing in the field windings 2 is increased and the output voltage of the synchronous machine 1 is eventually raised.
In the phase compensation circuit 64, the output of the amplifier circuit A 63 is compensated to increase the controlling stability, and the output signal from this phase compensation circuit 64 is added with outputs of various kinds of functional devices such as the power system stabilizer 7, V/Hz limiter 8 and the like in the adding circuit 65. The amplifier circuit B 66 amplifies the output of the adding circuit 65 properly and generates a signal proportional to the field current which is the output of the exciter 3 (i.e., the exciter 3 has an amplifying function as well). The output voltage of the synchronous machine 1 is controlled in the manner as above to the desired control value set by the reference circuit 62 within the AVR 6.
The amplifier circuit A 63 and B 66 and phase compensation circuit 64 are provided so that the output voltage of the synchronous machine 1 is agreed with the desired control value set by the reference circuit 62 in the AVR 6 as always as possible. Although it is ideal, needless to say, that the desired control value is coincident with the actual value at all times, the synchronous machine 1 itself has the time lag which produces the difference between the desired control value and actual value caused by an external disturbance to the controlling system, for example, when an accident happens at the output side of the synchronous machine 1, when the desired control value is changed, etc. Therefore, what the AVR 6 intends is to make control so as to remove the above difference as quickly as possible.
In the meantime, the V/Hz limiter 8 restricts the V/Hz value of the synchronous machine 1 under a predetermined value, with noting the fact that the current running in the synchronous machine 1 and coils of the transformers (not shown) is proportional to the V/Hz value. The V/Hz limiter 8 controls the current in the coils.
FIG. 2 is a block diagram of the transfer function representing the controlling characteristic of the AVR 6 and V/Hz limiter 8. Each of the exciter 3 and synchronous machine 1 is generally expressed by gains and first-order lag functions. In the AVR 6, amplifier circuit A 63, phase compensation circuit 64 and amplifier circuit B 66 are respectively represented by gains, first-order lead/lag functions and gains. Among those, the constants K.sub.E, T.sub.E of the exciter 3 and constants K.sub.G, T.sub.G of the synchronous machine 1 are fixed values the apparatus have. A gain K.sub.2 of the amplifier circuit B 66 is generally an output amplifying gain of the AVR 6 and therefore a fixed value the regulator has. On the other hand, the constants of the amplifier circuit A 63 and phase compensation circuit 64 are variable which can be set, by a variable resistor or the like. The voltage controllability of the AVR 6 is determined by these variable constants K.sub.1, T.sub.lead, T.sub.lag. These constants are designed beforehand on the basis of the constants of the synchronous machine 1 and exciter 3, but finally determined through individual adjustment. The V/Hz limiter 8 can be represented by gains and phase compensation similar to the AVR 6.
Because the conventional controller has the construction as above, when the frequency of the synchronous machine 1 is lowered, for instance, the V/Hz limiter 8 outputs a signal to lower the excitation. On the contrary, the AVR 6 outputs a signal to raise the excitation to make the voltage from the synchronous machine 1 constant. Therefore, it is necessary to set the respective gains to harmonize the V/Hz limiter 8 with AVR 6. It is also a drawback of the prior art that the desired V/Hz value of the V/Hz limiter 8 may not be perfectly coincident with the actual V/Hz, that is, an offset error increases.
Moreover, since the gain K.sub.1 of the amplifier circuit A 63 and the time constants T.sub.lead, T.sub.lag of the phase compensation circuit 64 should be set one by one, and if each constant of the exciter 3 or the synchronous machine 1 is different in the actual stage from the design stage, the above gain and time constants K.sub.1, T.sub.lead and T.sub.lag should be individually adjusted, so that the values vary depending on the adjusting people. The optimum constants cannot always be obtained and, finding the proper adjusting becomes difficult.